SUMMARY/ABSTRACT (PROJECT 1 - AN) Our overall goal in this proposal is to improve engraftment of anti-HIV gene modified hematopoietic stem/progenitor cells (HSPC) in vivo. HSPC based gene therapy holds great promise to provide long-term protection against HIV with the possibility of achieving a cure. However, studies to date have met with limited success largely due to the low efficiency of gene delivery into HSPC and subsequent hematopoietic reconstitution with anti-HIV gene modified cells. Overall we hypothesize that the efficiency of repopulation is important for the efficacy of HSPC-based gene therapy as apparent for the first case of HIV cure that was recently achieved by nearly complete bone marrow replacement by CCR5 deficient transplanted donor cells. However, unlike the first case of the HIV cured patient, complete bone marrow replacement by gene modified HSPC may not occur because of the limited efficiency of anti-HIV gene modified HSPC engraftment. Additionally, use of the intensive myeloablation procedure because of the presence of leukemia allowed in this first case of an HIV cure poses a high risk clinically. Therefore, in order to provide clinical benefit by HSPC gene therapy strategies, research is required to achieve more effective and safe procedures that can provide positive selection of genetically modified HSPC in patients. In this proposal, we will test a novel in vivo selection strategy that exclusively employs 6-thioguanine for both pre-conditioning and chemoselection of HPRT down-regulated genetically engineered HSPC, that is capable of enriching engraftment and long-term reconstitution of genetically engineered multi-pronged HIV resistant HSPC and progenies using a HIV infection established humanized mouse model. Our proposed research has a potential to break through the major obstacle in the field for HSPC gene therapy to enrich and efficiently repopulate gene-modified cells for the successful control of HIV disease without life long drug treatment and for HIV cure.